The present invention relates to an improved wood-type golf club head, and more particularly relates to improvements in construction and production of a wood-type golf club head including a shell made of fiber reinforced plastics.
One example of a wood-type golf club head including a shell made of fiber reinforced plastics is disclosed in Japanese Patent Publication No. sho. 51-21436. This golf club head includes a convitious shell made of fiber reinforced plastics and a synthetic resin core filled in the cavity of the shell. In production of this golf club head a foam synthetic resin crude core is first formed into the shape of the core, the shaped crude core is next covered, mostly manually, with a crude shell made of fiber reinforced plastics in order to form a crude golf club head, and the crude golf head is then encased within a proper mould for subsequent heating.
In the case of the above-described production process, however, troubles tend to start when there is any gap between the capacity of the mould cavity and the volume of the crude golf club head. Since formation of the crude shell on the crude core is usually carried out by manual operation, it is difficult to constantly set the volume of the crude golf club head to a correct value. As a consequence, unavoidable variation in volume of the crude golf club head produces the above-described gap. Presence of such a gap between the capacity of the mould cavity and the volume of the crude club head develops variation in pressure to be applied to the crude golf club head placed in the cavity of the mould at mould fastening. When the fiber reinforced plastic crude shell is made up of several super-imposed layers, insufficient pressure tends to connect to poor inter-layer combination and development of voids between the layers and/or on the surface of the produced golf club head. Excessive pressure pushes some of the crude shell into gaps between mating surfaces of mould pieces at mould fastening to produce undesirable projections or flashes on the surface of the produced golf club head. For better appearance of the product, these flashes have to be removed after moulding of the golf club head. Such removal tends to deteriorate the quality of the product and, in worst cases, destroys fibers in the fiber reinforced plastic shell whilst lowering the strength of the entire golf club head.
In order to make up for the above-described disadvantage inherent to the conventional process, a new process of production was proposed in U.S. patent application Ser. No. 516,800 filed on 25th July, 1983, now abandoned. In the process of this prior proposal, a cavitious crude core is made of thermoplastic synthetic resin, the crude core is next covered with a crude shell made of fiber reinforced plastics to form a crude golf club head, and the crude golf club head is then subjected to heat pressing in a mould in order to press the crude shell against the mould cavity wall via inflation of the crude core. This inflation of the crude core at head pressing is caused either by thermal expansion of air initially confined in the crude core or by later introduction of pressurized medium into the crude core placed in the mould cavity.
In the case of a golf club head produced by this proposed process, the crude core is pressed against the crude shell due to its own inflation and the core in the produced golf club head is cavitious in construction. In other words, there is nothing else in the construction of the golf club head to press the core to the shell. Depending on the rate of inflation at heat pressing, there is a danger that, in the construction of the produced golf club head, the core is poorly bound to the fiber reinforced plastic shell. Such poor binding tends to allow separation of the core from the shell at hitting of balls by the golf club head. When a golf club is swung under such a separated condition of the components, impulsive touch and impingement of the core with the fiber reinforced plastic shell generates harsh noises which get on user's nerves greatly and, as a consequence, lower the commercial value of the product.
In addition to such a separation problem resulted from ill production, the cavitious construction of the produced golf club head has its own demerits. Due to shock at hitting balls, elastic deformation of the cavitious construction is liable to generate vibrations in the fiber reinforced plastic core which bar smooth transmission of striking energy to balls, thereby significantly decreasing the distance of flight. Further, keen sounds are generated at hitting balls. In addition, regional concentration of shock at hitting ball tends to occur during its transmission through the face side section of the fiber reinforced plastic shell. When the core is made of foam synthetic resins which is in general fragile and poor in buckling strength, the above-described shock concentration causes development of fine cracks in the peripheral section of the core, thereby lowering durability of the golf club head.